Neuroscientist John Cacioppo, who has spent much of his career working on loneliness, defines “loneliness” as “perceived social isolation.” Similarly, Masi, et al. (following Russell, et al. 1980) define “loneliness” as “the discrepancy between a person’s desired and actual social relationships.” As Masi, et al., point out, there is a distinction to be made between loneliness, on the one hand, and social isolation, on the other, although the two phenomena may indeed often go together. Whereas social isolation “reflects an objective measure of social interactions and relationships,” loneliness “reflects perceived social isolation or outcast.” Following Peplau & Perlman 1982 and Wheeler, et al. 1983, they go on to point out that “loneliness is more closely associated with the quality than the number of relationships.” (It’s important and timely to note that the 2016 Nobel Laureate in Literature, Bob Dylan, brought out one application of this conceptual distinction in his song, “Marchin’ to the City,” when he sang: “Loneliness got a mind of its own / The more people around the more you feel alone.”)

The health risks posed by loneliness are several and can be severe. Loneliness can contribute to increased risk of coronary heart disease, cardiovascular disease, and stroke. In a 2016 systematic review and meta-analysis in Heart, Valtorta, et al., reported that “poor social relationships were associated with a 29% increase in risk of incident CHD [coronary heart disease] and a 32% increase in risk of stroke.” And in a March 2015 meta-analysis in Perspectives on Psychological Science, Holt-Lunstad, et al., reported that a substantial body of evidence supports the following two claims:

Loneliness puts one at greater risk for premature mortality. In particular, “the increased likelihood of death was 26% for reported loneliness, 29% for social isolation, and 32% for living alone.”

The heightened risk for mortality due to “a lack of social relationships” (whether reported loneliness, social isolation, or living alone) is greater than the risk due to obesity.